No audio from group tracks

Using Cubase 8.5 for this current project, but have been having this issue since 7/8
Basically, group tracks are self muting their outputs somehow. EG, I send a few channels of backing vocals to a group, apply inserts and EQ to the group… then I might automate the group, apply a mute or two… usual stuff.
Then, every now and then, on playback, the channel just goes dead - like it’s muted (but it’s not). Sometimes I randomly hit the solo and mute buttons, or turn off the inserts… and it will come back for a little while. But usually, I have to quit and restart to get it to come back to normal. It’s driving me nuts. Often I will still be able to hear the hiss of the group if there’s a flanger insert or CLA bass insert… but no audio.
If I send the audio from the channel directly to the stereo output and it works!
I’ve resorted to exporting the solo’d group tracks as a mix down and importing it back into the project and routing it to the stereo output in the mix. Obviously this is very limiting.
I’ve contacted Steinberg a few times but they have only ever said it’s not an issue - even though I’ve read threads where others have this issue too.
Any thoughts?
Cubase 8.5 (also 8 and 7 - I have 9 but haven’t really used it yet)
Windows 7
I7 processor
12gb ram
A&H Zed R16 desk/interface